Microprocessor interface apparatus having a boot address relocator, a request pipeline, a prefetch queue, and an interrupt filter

ABSTRACT

A processor interface chip and a maintenance diagnostic chip are provided coupled with two microprocessors designed to be run in tandem. The processor interface chip includes logic for interfacing between the microprocessors and a main memory, logic for pipelining multiple microprocessor requests between the microprocessors and main memory, logic for prefetching data before a microprocessor issues a read request, logic for allowing a boot to occur from code anywhere in physical memory without regard to the microprocessors&#39; fixed memory location for boot code, and logic for intelligently limiting the flow of interrupt information over a processor bus between the microprocessors and the processor interface chip. The maintenance diagnostic chip includes logic to halt either of the microprocessors if an error is detected, and read out the state of the microprocessors and a secondary cache attached to the microprocessors, before the state of the microprocessors at the time of the fault changes to a different state which might hide evidence of the cause of the fault.

This is a division of application Ser. No. 08/088,562 filed Jul. 6,1993, now U.S. Pat. No. 5,735,001.

BACKGROUND OF THE INVENTION

The present invention relates to the field of processor interfacecircuitry. More specifically, in one embodiment the invention providesan improved interface between a microprocessor, or a set ofmicroprocessors, and other processor circuits.

In many cases, a microprocessor can be designed to run faster thanexternal components with which it communicates. Unfortunately, themicroprocessor often cannot proceed until a particular action is takenby the external device, and thus the performance of the processor systemin which the microprocessor is used is adversely affected. One reasonfor this bottleneck is that communication between two circuits on thesame integrated circuit, or chip, is generally faster than communicationbetween two circuits separated by an inter-chip bus or other interface.Thus, one solution to the need for faster interaction with themicroprocessor is to place more circuitry on the microprocessor chip,such as data and instruction caches. However, adding higher-levelcomponents on chip with the microprocessor make diagnosing errors muchmore difficult. This is because by the time an internal error isdetected within the microprocessor and percolates out of the chip to adiagnostic system, the diagnostic system has much less time toinvestigate the cause of the error before the continued operation of themicroprocessor changes the state of its internal circuits to the pointwhere the state at the time of the error is no longer known. Forexample, if a data error occurs deep inside the microprocessor, but isdetected and apparently fixed by logic inside the microprocessor beforebeing output, external circuits may act on that data as being valid datathereby corrupting the processor system.

Another problem with processor systems is the microprocessor bus, overwhich most of the microprocessor requests and responses to thoserequests pass. The microprocessor bus carries write requests, along withthe data to be written, read requests, read and write responses back tothe microprocessor, and interrupt signals into the microprocessor. Thistraffic over the bus often limits the speed at which data can beaccepted from, and provided to, the microprocessor.

From the above it is seen that an improved interface to a microprocessoris needed.

SUMMARY OF THE INVENTION

In one embodiment of a processor interface system according to thepresent invention, a processor interface chip and a maintenancediagnostic chip are provided, coupled with two microprocessors designedto be run in tandem. The processor interface chip includes logic forinterfacing between the tandem microprocessors and a main memory, logicfor pipelining multiple microprocessor requests between themicroprocessors and main memory, logic for prefetching data before amicroprocessor issues a read request for the prefetched data, logic forallowing a boot to occur from boot code anywhere in physical memorywithout regard to the microprocessors' fixed memory location for bootcode, and logic for intelligently limiting the flow of interruptinformation over a processor bus between the microprocessors and theprocessor interface chip. The maintenance diagnostic chip includes logicto halt either of the microprocessors if an error is detected, and readout the state of the microprocessors and a secondary cache attached tothe microprocessors, before the state of the microprocessors at the timeof the fault changes to a different state which might hide evidence ofthe cause of the fault.

A further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an overview of a processor systemaccording to the present invention, including two microprocessors, theprocessor interface chip (PIC) and the maintenance diagnostic chip(MDC);

FIG. 2 is a timing diagram illustrating the interaction between themicroprocessors and the maintenance diagnostic chip following a fault;

FIG. 3 is a block diagram showing the PIC in further detail, including aboot address translation circuit, a prefetch queue, an interrupt filterand a request pipeline;

FIG. 4 is a block diagram showing the prefetch queue in greater detail;

FIG. 5 is a block diagram showing the boot address translation circuitin greater detail;

FIG. 6 is a memory map of a physical memory addressed by themicroprocessors;

FIG. 7 is a block diagram showing the interrupt filter in greaterdetail; and

FIG. 8 shows an example of a three level interrupt hierarchy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an overview of a processor system 10 according to the presentinvention. Processor system 10, in one preferred embodiment, comprisestwo microprocessors, and several special purpose chips on a processorboard, however other configurations are possible, such as includingseveral circuits shown on one chip or providing several chips forindividual functions. Processor system 10 is shown in FIG. 1 with twomicroprocessors 12(0,1), a Maintenance and Diagnostic Chip (MDC) 14, aProcessor Interface Chip (PIC) 16, a Memory Interface Chip (MIC) 20, amain memory 22, a secondary cache 30. Other components which are notshown may be included.

Several busses interconnecting components are also provided. A processorbus (Pbus) 18 couples PIC 16 and the microprocessors 12(0,1); amaintenance bus (Mbus) 24 couples MDC 14 to PIC 16 and MIC 20 andcarries diagnostic commands and data to and from MDC 14; an internal bus(Ibus) 26 couples PIC 16 to MDC 14 and MIC 20; and a secondary cache bus28 couples microprocessors 12(0,1) to secondary cache 30.

For reliability, several busses use information lines and check lines,where the information lines carry the independent signals for which thebus exists (such as data, instructions, addresses, control signals,etc.), and the check lines carry check signals which are a function ofthe values on the information lines and a check function such as checksum, parity, or other error-correcting code (ECC) functions. Forexample, Ibus 26 comprises, in part, 32 signal lines and four paritylines, where each of the parity lines carries an even parity check ofeight of the 32 information signal lines. In FIG. 1, some busses are notshown with check lines separated from information lines.

Microprocessors 12 can operate in a "complete master" mode, where onemicroprocessor controls the information lines and the check lines ofboth Pbus 18 and SC bus 28, or they can operate in a "lock-step" or a"partial master", mode where each microprocessor 12 controls on bus ofPbus 18 and SC bus 28. For reliability, both microprocessors 12 read thebusses and execute the same instructions, but only one, the master of abus, drives the information lines of the bus, while the othermicroprocessor 12 monitors the information lines and compares the valueson those lines with what it would have driven on those lines (its"potential" output) if it were the master for that bus. If thenon-master microprocessor disagrees with what is on the informationlines of the bus, it triggers an "output miscompare" fault, which isexplained in connection with FIG. 2. For further reliability, the masterof a bus does not drive the bus check lines, the non-master does. Thisway, if the microprocessors are operating normally, but get out of stepwith each other, other devices on the bus will notice the error, as thecheck lines will not likely be correct.

FIG. 1 shows one microprocessor, 12(0), as being the master of SC bus 28and is labelled "SC Master" (Secondary Cache Master), while the othermicroprocessor, 12(1), is the master of Pbus 18, and is labelled "SIMaster" (System Interface Master--Pbus 18 is the "system interface" inthis case). Thus, when operating in lock-step, SC Master 12(0) drivesthe information lines of SC bus 28 (Address/Data/ECC) and monitors thecheck lines of SC bus 28 (Adr/Cnt parity--address and control lineparity), while SI Master 12(1) drives the check lines and monitors theinformation lines of SC bus 28. Conversely, SI master 12(1) drives theinformation lines of Pbus 18 (address/data) and monitors the check linesof Pbus 18 (ECC/parity), while SC Master 12(0) drives the check linesand monitors the information lines of Pbus 18.

In some embodiments, one microprocessor might be both the SC Master andthe SI Master, in other words, a "complete master" with the othermicroprocessor is a "complete listener", duplicating the operation ofthe complete master, but not driving any lines except possibly its faultline, which it does not share with the complete master. While the ECClines of SC bus 28 are actually check lines, they are grouped with theinformation lines. This is because the ECC lines are used by secondarycache 30 to do error checking there, and if all the lines into secondarycache 30 come from the same microprocessor, the secondary cache can runfaster without worrying about slight variations in timing which mightoccur between the two microprocessors 12(0,1), thus causing the addressand data to arrive at the secondary cache offset in time with the ECCsignals. Such timing variations might be caused by process variations increating the microprocessors.

In a preferred embodiment, microprocessors 12(0,1) are R4400microprocessors manufactured by the MTI division of Silicon Graphics,Inc.

In addition to the busses (Ibus, Pbus, SC bus, Mbus), other signal linesexist between various components. An error signal line from PIC 16 toMDC 14 carries a CE/UCE error signal which indicates that the PIC hasencountered a correctable or uncorrectable error on the Pbus. If checkvalues on the check lines of the Pbus do not correctly reflect the checkfunction for the information values on the information lines of thePbus, then the PIC asserts the CE/UCE error signal.

Three lines, FAULT*, RESET*, and MODEIN are provided between eachmicroprocessor 12 and the MDC. The FAULT* line is driven low by themicroprocessor when it detects a fault. The RESET* and MODEIN lines aredriven by the MDC 14. The RESET* signal is an active low signal whichholds the microprocessor in a reset state when the signal is low, andthe MODEIN signal controls a mode of the microprocessor. Where theRESET* signal is being asserted (held low by MDC the MODEIN signalcontrols the meaning of signals on the FAULT* line.

FIG. 2 shows the interaction of the FAULT*, RESET*, and MODEIN signalsin more detail. FIG. 2 is a timing chart, which is divided into periodslabelled A-K, which are not equal spans of time, but which differentiatedifferent periods of activity on these signal lines. These periods arebriefly described in Table 1.

Per. Description

A SI Master and/or SC Master asserts their FAULT, line, which isdetected by the MDC.

B MDC asserts RESET* signal for both microprocessors.

C MDC continues to assert both RESET* signals (i.e., holding the RESET*lines low) and asserts both MODEIN signals (by driving the MODEIN lineshigh), in which state the FAULT* lines of each microprocessor indicate(by going low) whether that microprocessor believes an output miscomparewas the first fault to trigger the fault.

D MDC continues to assert both RESET* signals and drives both MODEINlines high, in which state the FAULT* lines of each microprocessorindicate (by going low) whether that microprocessor believes an inputfault occurred.

E MDC releases the RESET* signal for the SI Master only, at which pointthe SI Master becomes a complete master of SC bus 28, Pbus 18, and thecheck lines for both busses.

MDC reasserts the RESET* signal for the SI Master and deasserts theRESET* signal for the SC Master, at which point the SC Master becomes acomplete master. At the end of this period, MDC reasserts the RESET*signal for the SC Master.

G MDC deasserts the RESET* signal for both microprocessors, and theyboth run as partial masters. At the end of this period, MDC reassertsboth RESET* signals.

H MDC deasserts the RESET* signal for the SI Master, and holds(continues to assert) the RESET* signal for the SC Master. In thisperiod, the SI Master is a complete master.

I MDC reasserts the RESET* signal for the SI Master and deasserts theRESET* signal for the SC Master. In this period, the SC Master is acomplete master.

J MDC reasserts both RESET* signals for some finite time period.

K MDC deasserts both RESET* signals, and the microprocessors come up aspartial masters.

TABLE 1. Processing Periods Following a Fault

The timing chart (period A) begins with either the SC Master (shown as12(0) in FIG. 1) or the SI Master (shown as 12(1) in FIG. 1) detecting afault, and asserting its FAULT* line by driving it low. This signal ispicked up by MDC 14. For some errors, such as where the PIC drives Pbus18 with incorrect parity, both microprocessors 12 might assert theirseparate fault lines. For other errors, only one microprocessor 12 mightdetect the error.

In any case, when a fault occurs, MDC 14 must quickly determine thestate of microprocessors 12. The state of microprocessors 12 is thevalues of its internal registers and flags. For complete diagnostics,MDC 14 must also obtain the contents of the primary caches of eachmicroprocessor 12 and the contents of the shared secondary cache 30 (seeFIG. Where microprocessors are used in which instructions and data arecached separately, the primary caches include a primary instructioncache and a primary data cache.

Once the MDC receives the FAULT* signal, the MDC asserts the RESET* lineof both microprocessors (period B). When the RESET* signal is asserted,the microprocessor goes into a state where all of its outputs aretri-state outputs except the FAULT* line. This allows other lock-stepmicroprocessors to completely control the busses without interference.Microprocessors 12 contain internal logic in which a bit can be set andremembered after a reset. This bit indicates whether microprocessor 12is an SI Master or an SC Master when in the partial master mode. Eachtime microprocessor 12 is reset and the reset is held for at least somepredetermined amount of time, the master mode of the microprocessortoggles between the complete master mode and the partial master mode.

In addition to holding microprocessors 12 in a reset state, the MDC alsosends a hold signal over Mbus 24 to preserve the state of the devicescoupled to Ibus 26.

While in a reset mode, logic within microprocessor 12 provides furtherfault indications on the FAULT* line which depend on the state of theMODEIN line (periods B,C). When the MODEIN line is low, the FAULT* lineis low (logical 0) if an output miscompare first triggered the faultwhich resulted in the initial FAULT* pulse. As explained above, anoutput miscompare fault is expected from one microprocessor when a linebeing driven by the other microprocessor is being driven to a valuedifferent than the one microprocessor's potential output. Unless theoutput logic is faulty, a microprocessor cannot logically detect anoutput miscompare on the lines it is driving. Since each microprocessor12 is a master for some lines, the output miscompare indications willindicate the likely lines on which the miscompare occurred, on either SCbus 28 or Pbus 18. The FAULT* line will be driven low (logical 0) by themicroprocessor detecting the error if an output miscompare was detected.

Next, in period C, the input fault history bit from each microprocessor12 is read from the FAULT* lines. When the MDC, which keeps the RESET*lines low, drives the MODEIN lines high, microprocessors 12 output aninput fault history bit on their FAULT* lines, driving the lines low toindicate an input fault. The input fault history bit does not indicatethat the input fault was the first fault to occur, but just that aninput fault did occur at some time since the input fault bit was reset.

After the fault is detected by the MDC, the MDC loads diagnostic codeinto memory 22 at the boot location for the microprocessors. The bootlocation is the first instruction location read by the microprocessorupon reset (although, as explained later, this address might berelocated to a physical memory location by boot address relocator 194).

Once the diagnostic code is loaded, the RESET* signal is deasserted onmicroprocessor 12(1) (the SI Master in a partial master mode), and itruns as a complete master, as explained above. The diagnostic codecauses microprocessor 12(1) to dump its state and the contents of itsprimary cache (period E). The diagnostic code is usually written suchthat the primary cache is not used while running this code, so that itcan be read without being destroyed first.

Since the RESET* line on microprocessor 12(0) is still asserted, theoutput lines of that microprocessor are tri-stated. This allowsmicroprocessor 12(1) to run the diagnostic code which causes it to makethe dump without interference by microprocessor 12(0). The dumped datacan then be picked up by the PIC and passed to MIC 20 to store in memory22 for later analysis. Of course, given that a fault has occurred,either of the microprocessors 12 might not behave properly and mightinterfere with the collection of diagnostic data.

At the end of period E, the RESET* line on microprocessor 12(1) is againasserted. Starting in period F, the RESET* line for microprocessor 12(0)is deasserted and it begins to run diagnostic code to dump its state andprimary cache. Once both microprocessors 12 have dumped their state andprimary caches, secondary cache 30 needs to be dumped.

Since MDC 14 does not connect directly to SC bus 28, secondary cache 30must be dumped through one of the microprocessors 12. MDC 14 couldconnect directly to SC bus 28 for this purpose, but connecting anotherdevice to SC bus 28 would slow its response time, so MDC 14 readssecondary cache 30 via a microprocessor 12. Until the problem causingthe fault is diagnosed, it is unknown which microprocessor 12 is faulty,if either, so secondary cache 30 is read out using both microprocessorsoperating in lock-step, and then using each microprocessor separately,to provide three copies of the secondary cache for the analysis.

If an output miscompare occurs while secondary cache 30 is being readout of the lock-stepped microprocessors 12(0,1), it is ignored. Sinceeach microprocessor 12 is set to come up in a partial master mode whenits reset line is released, secondary cache 30 is first read in thelock-step (two partial masters) mode (period G). Note that in order forthe SC Master to be a partial master, the SC Master must have been resetfor some finite time between periods F and G, as is shown in FIG. 2.

In period G, both microprocessors 12 run the same code which instructsthem to dump the contents of secondary cache 30 to PIC 16, which passesit to MIC 20 for storage in memory 22. If an output miscompare occursduring this dump, a FAULT* line might be asserted, but it is ignored bythe MDC (although it may be noted and logged by the MDC). Of course, thediagnostic code is usually written such that the secondary cache is notused while running this code.

In period H, microprocessor 12(1), as a complete master, dumps thecontents of secondary cache 30, and in period I, microprocessor 12(0),as a complete master, dumps the contents of secondary cache 30.

Period J illustrates the finite time period which is required for thereset line to toggle the partial/complete master mode in microprocessor12(0). Once the states and the primary caches of each microprocessor 12and secondary cache 30 have been dumped, MDC 14 can proceed to analyzethe dump, or could cold-reset microprocessors 12 by deasserting theRESET* lines (period K).

FIG. 3 shows PIC 16 in greater detail. The PIC contains many elementsnot shown, and is roughly divided into a Pbus interface section 180, anIbus interface section 182, a request pipeline 195, a prefetch queue196, and an interrupt filter 198.

Pbus interface section 180 contains logic for reading data from Pbus 18and for outputting data from several other components of PIC 16 ontoPbus 18. The components of Pbus interface section 180 shown in FIG. 3are a multi-bit input driver 202 coupled to Pbus 18, a Pbus inputregister 250 coupled to input driver 202, a prefetch queue monitor 254coupled between a command path output by input register 250 and prefetchqueue 196, a Pbus output multiplexer 246 with a select input receivedfrom a multiplexer controller 256 and an output coupled to a Pbus outputregister 248, which in turn is coupled to an input of a multi-bit outputdriver 252. The output of register 250 is split into a command path anda write data path, with the commands going to prefetch queue monitor 254and request pipeline 195, and the write data going to a write buffer 208of request pipeline 195.

Ibus interface section 182 contains logic for reading data from Ibus 26and for outputting data from several other components of PIC 16 ontoIbus 26. The components of Ibus interface section 182 shown in FIG. 3are a second multi-bit input driver 242 connected to Ibus 26, an Ibusinput register 244 coupled to input driver 242, an Ibus outputmultiplexer 210 with a select input received from a second multiplexercontroller 212 and an output connected to a boot address relocator 194,which outputs to an Ibus output register 240, which is in turn connectedto an input of a second multi-bit output driver 214. The output ofregister 244 is split into two outputs, with one output for interruptsgoing to interrupt filter 198 and the other output for memory readresponses going to prefetch queue 196.

Request pipeline 195 will now be described in further detail withreference to FIG. 3. Following the description of request pipeline 195,prefetch queue 196 is described with reference to FIG. 4 and interruptfilter 198 is described with reference to FIG. 7.

Request pipeline 195 comprises a pipeline tail register PTAIL (DMI)204(1), a pipeline head register PHEAD (DMO) 204(2), a pipelinecontroller 206, and a write buffer 208. Write buffer 208 includes afull/empty flag 209 for indicating whether the write buffer is full orempty. Pipeline controller 206 also maintains a register (not shown)indicating how full or empty write buffer 208 is, as well as aprogrammable threshold (also not shown) which can be compared to theregister for deciding whether to send the incoming write request to Ibus26 or to wait for write buffer 208 to fill further.

The command portion of a microprocessor request, such as an indicationthat the request is a memory read request and an indication of theaddress to be read, is either stored in the pipeline tail, the pipelinehead, or is passed directly to multiplexer 210 and onto Ibus 26. In somecases, a memory read request might not reach the request pipeline, butwill be routed to the prefetch queue 196. This occurs when the data tobe read by the microprocessor is already present in the prefetch queue.This condition is detected by comparing the address portion of the readrequest with the address tags stored for the buffers of the prefetchqueue 196. If the read request does enter request pipeline 195, however,it can be passed directly through request pipeline 195, or stored ineither PTAIL or PHEAD. If both PTAIL and PHEAD are empty and Ibus 26 isfree, the read request passes directly to multiplexer 210.

If Ibus 26 is busy, the read request is placed in PHEAD and pipelinecontroller 206 returns a request acceptance to the microprocessor overPbus 18, so that the microprocessor Call continue. If PHEAD is occupied,the read request is placed in PTAIL, and moved along to PHEAD when PHEADis free; again, pipeline controller 206 returns a request acceptance tothe microprocessor. If PTAIL is also occupied, then the read request isheld in input register 250, and pipeline controller does not send back arequest acceptance. Until pipeline controller 206 sends back a requestacceptance (when the read request is finally loaded into PTAIL or PHEAD,or sent to Ibus 26), the microprocessor avoids using Pbus 18 to sendmore requests.

Write requests are placed in PTAIL 204(1) and the write data iscollected off Pbus 18 into write buffer 208. When write buffer 208 isfull, full/empty flag 209 is set to "full". When the threshold amount ofdata is loaded into write buffer 208, pipeline controller 206 moves thewrite request from PTAIL to PHEAD. If Ibus 26 is available, the writerequest then moves there, and when the write request and accompanyingdata is sent over the bus, the full/empty flag is set to "empty". In theembodiment shown in FIG. 3, only one write request can be in the requestpipeline at one time, but other embodiments, with different constraintsas to bus performance and allocated chip area might have multiple writebuffers or more than two pipeline stages 204.

In order to avoid sending obsolete data to the microprocessor, prefetchqueue monitor 254 monitors the write requests, and signals prefetchqueue 196 to invalidate any data the prefetch queue may have alreadyretrieved from the memory locations which are to be written by the writerequests. A dotted line connecting pipeline control 206 and multiplexercontroller 212 is used to indicate that pipeline controller 206 signalsto multiplexer controller 212 which, if any, output of request pipeline195 is to be output onto Ibus 26.

Because Ibus 26 is only accessed when a write request and its write dataare complete in PIC 16, Ibus 26 is used more efficiently. Several buscycles of Pbus 18 are needed to get all the write data, so Ibus 26 isnot used until write buffer 208 is at least filled to the programmablethreshold stored in pipeline controller 206.

FIG. 4 shows prefetch queue 196 in greater detail. Prefetch queue 196comprises a control state machine 312, a most recently used register(MRU) 310, and two buffers (PFQ0, PFQ1) 226(0,1). MRU 310 points to themost recently used buffer 226. Each buffer includes storage for eightdata words and associated parity bits, an address tag register 302, avalidity flag 304, a hard abort flag 306, and an uncorrectable memoryerror (UCME) flag 308. Control state machine 312 is coupled to Ibus 26and Pbus 18 through bus interface registers, and is coupled to read andwrite the storage areas and various flags of each buffer 226. Controlstate machine 312 also receives signals from prefetch queue monitor 254which are used to provide prefetched data to Pbus 18 and to invalidatedata which has been written after being read into a buffer 226. Controlstate machine 312 includes an output over which memory read requests aremade via multiplexer 210.

Prefetch queue 196 operates as follows. For non-prefetch operations,PFQ0 is used as an Ibus buffer. For prefetch operations, control statemachine 312 is made aware of a read address of a read request, eitherthrough monitor 254 or from the data coming from Ibus 26. Control statemachine 312 then makes a request for the data from the addressesfollowing the block which was actually requested, or makes a singlerequest over Ibus 26 for twice as much data as was requested in the readrequest. The block of data which as actually requested is sent along tothe microprocessor over Pbus 18, and the other half is stored in aprefetch queue buffer 226 until requested.

Suppose MRU 310 indicated that PFQ1 was most recently used, and thatboth validity flags 304(0,1) were reset. When a read request is sent toPIC 16, the read request cannot filled by the prefetch queue, so therequest is put on Ibus 26 by request pipeline 195. A typical readrequest asks for eight words, but to fill the prefetch queue, therequest on Ibus 26 asks for sixteen words. If the sixteen word requestwould cross a DRAM (dynamic random access memory) page boundary, therequest is sent out as two eight-word requests. When the 16 words arereturned, eight are sent on to fill the request, and the other eightwords are stored in PFQ0 (the oldest buffer). MRU 310 is toggled topoint to PFQ1, tag register 302(0) is updated with the address of thelatter eight words stored in PFQ0, and validity flag 304(0) is set.

If, during the read of the first eight words, a hard abort error or UCMEoccurred, an indication of that condition is passed on to themicroprocessor. However, if the error occurred in the latter eightwords, the indication is not passed on to the microprocessor until themicroprocessor actually requests the data which causes the error. If ahard abort is caused by reading the latter eight words, the hard abortflag 306(0) is set, and if an UCME occurred reading the latter eightwords, UCME flag 308(0) is set.

When PFQ monitor 254 indicates that a read request was issued for anaddress matching one of the tag registers 302(0,1), that request isfilled from the prefetch queue, and the buffer 226 which filled therequest is marked invalid by resetting its validity flag 304. Anytimethere is an invalid buffer 226, another eight words could be fetched, sothat prefetch queue 196 is rarely a bottleneck for data flow. When awrite request is sent to request pipeline 195, PFQ monitor 254 suppliesthe write address to control state machine 312, which compares it to tagregisters 302(0,1). If the write address matches either tag register,control state machine 312 resets the validity flag 304 for the buffer226 associated with the matching tag register.

FIG. 5 shows boot address relocator 194 in greater detail. In FIG. 5,boot address relocator 194 comprises a boot exception vector indicatorregister (BEV PIC) 218 for storing a bit indicating whether or not bootaddress relocation is to be done, two-bit boot address register 220, sixtwo-input multiplexers 402(1..6), an AND gate 400 and an exclusive OR(XOR) gate 404. An address comprises 32 bits and four bits of parity,one parity bit for each byte (8 bits) of address. The parity bits areeven parity, so that dmo₋₋ r₋₋ pb[3] is an XOR of dmo₋₋ r[31:24], dmo₋₋r₋₋ pb[2] is an XOR of dmo₋₋ r[23:16], and so on.

Register 220 and BEV₋₋ PIC 218 can be set in a number of ways, such asbeing controllable by MDC 14. One way MDC 14 inserts values intoregisters is by inserting the desired values into scan data and runninga scan on the registers, reading out their current content whileinserting new content.

Boot address relocator 194 has a bus input, a bus output, and an inputto indicate whether the content of the bus is an address which will beplaced on Ibus 26. If the content of the bus is not an address whichwill be placed on Ibus 26, the data is passed through boot addressrelocator without modification. AND gate 400 has two inputs, one fromBEV₋₋ PIC 218 and the other from an input which indicates if the inputis an address for Ibus 26. If both are true, then AND gate 400 outputs alogical 1 (SELECT=1) to the select inputs of multiplexers 402(1..6),which causes the relocation of the address on the bus. Otherwise, if ANDgate 400 outputs a logical 0 (SELECT=0), the data on the bus passesthrough boot address relocator 194 unchanged.

Table 2 shows the logic of the multiplexers and its effect on the bitsof the address lines.

                  TABLE 2                                                         ______________________________________                                        Boot Relocation Addressing                                                    Line(s)    SELECT = 0   SELECT = 1                                            ______________________________________                                        31:24      dmo.sub.-- a[31:24]                                                                        0                                                     23:22      dmo.sub.-- r[23:22]                                                                        0                                                     21         dmo.sub.-- r[21]                                                                           boot.sub.-- adr[1]                                    20:17      dmo.sub.-- r[20:17]                                                                        0                                                     16         dmo.sub.-- r[16]                                                                           boot.sub.-- adr[0]                                    15:08      dmo.sub.-- r[15:08]                                                                        dmo.sub.-- r[15:08]                                   07:00      dmo.sub.-- r[07:00]                                                                        dmo.sub.-- r[07:00]                                   parity 3   xor(dmo.sub.-- r[31:24])                                                                   0                                                     parity 2   xor(dmo.sub.-- r[23:16])                                                                   xor(dmo.sub.-- r[21:16])                              parity 1   xor(dmo.sub.-- r[15:08])                                                                   xor(dmo.sub.-- r[15:08])                              parity 0   xor(dmo.sub.-- r[07:00])                                                                   xor(dmo.sub.-- r[07:00])                              ______________________________________                                    

FIG. 6 shows a memory map of 32-bit addresses from 0×00000000 to0×FFFFFFFF which illustrates the effect of boot address relocation. Inone embodiment of a processor system, the operating system expectsphysical memory at the low addresses starting at 0×00000000, andspanning 32, 64, 128, or 256 megabytes (MB). However, microprocessor 12expects to find boot code at 0×1FC00000. Both these needs can be met byeither using a memory of at least 508 Mb to span the space from0×00000000 to 0×1FC00000, or by adding a small memory at 0×1FC00000which contains code including a jump to a location in the physicalmemory. While microprocessor 12 is running, it can perform virtualmemory address translations with its internal translation look-asidebuffer (TLB), but following a reset it has not yet configured itself forvirtual memory operations. The boot code assists in this setup, so itmust be located in, or relocated to, an address where microprocessor 12expects it.

Four sections of physical address space (labelled 00, 01, 10, and 11)are available for boot code. Since these sections are all located withinthe first 4 MB of memory, they are all located in the installed physicalmemory of the embodiment discussed above. Boot address relocator 194relocates addresses to one of these four sections, where the particularone of the four sections is determined by the contents of the bootaddress register 220, which is labelled as BOOT₋₋ ADR[1:0].

FIG. 5 shows multiplexer 402(3) in a dotted outline to indicate that itis not really needed since bits 20:17 are all zeroes in a boot addressin the above example anyway. In this case, multiplexer 402(3) can beeliminated to save chip real estate.

FIG. 7 shows the details of interrupt filter 198, which includes aninterrupt image register 452, a third priority level interrupt register460, a third priority level interrupt mask 462, a second priority levelinterrupt register 464, a second priority level interrupt mask 466, amulti-line comparator 470, an output driver 480, and an input driver482. FIG. 7 also shows an interrupt register 450 within microprocessor12.

An interrupt input path is shown, where an interrupt propagates fromIbus 26, and in order, through register 460, mask 462, register 464,mask 466, and onto internal bus 490. Five lines of internal bus 490 areprovided to interrupt image register 452, which has a five-line outputto comparator 470. A comparator output of comparator 470 is coupled toan output enable of driver 480. The input to driver 480 is internal bus490. The output of driver 482 is provided to the registers and masks,while the input of driver 482 and the output of driver 480 are coupledto Pbus 18 through Pbus interface section 180 (not shown; see FIG. 3).Interrupt register 450 of microprocessor 12 is coupled to the Pbus aswell.

In operation, interrupts from Ibus 26 are filtered so that the onlyinterrupts which reach microprocessor 12 are interrupts which wouldalter the contents of interrupt register 450, however, microprocessor 12is free to query or change any interrupt register or mask in interruptfilter 198.

FIG. 8 shows registers 460, 464 in more detail. Although not shown,masks 462, 466 contain a bit for each interrupt of registers 460, 464,respectively.

Interrupts received over Ibus 26 are stored in register 460 according totheir interrupt number. For some interrupts, a value is passed alongwith the interrupt number and this value is stored along with anindication of the setting of the interrupt, which is either in a setstate or in a reset state (i.e., a cleared interrupt). These incominginterrupts are masked and prioritized according to a priority scheme,such as that shown in FIG. 8. The priority of an interrupt is determinedby its interrupt number and by its priority grouping. For example, amongthe grouping of interrupts int₋₋ io[19:10], int₋₋ io[19] has the highestpriority. Therefore, if int₋₋ io[19] and lower priority interrupts areset by incoming interrupt events over Ibus 26, only the int₋₋ io[19]interrupt will be passed on to trigger the int₋₋ a[15] interrupt at thenext priority level. The number "19" might also be stored in int₋₋ a[15]so that the number of the interrupt within the priority group causingthe int₋₋ a[15] interrupt can be readily determined. When int₋₋ io[19]is cleared, then the next highest priority interrupt would propagate upto int₋₋ a[15].

The mask registers contain flags for each interrupt, and if the mask bitis set, that interrupt is not sent on to the next level. Therefore, ifthe mask bit for int₋₋ io[19] is set, an int₋₋ io[19] interrupt wouldnot be passed on to int₋₋ a[15] even though it is the highest priorityinterrupt.

Likewise, interrupts are prioritized at the second level, which reducesthe interrupts to five at the first level. However, even with thisnarrowing of the number of different interrupts, the frequency ofinterrupts is not necessarily reduced, since an interrupt will oftenpropagate all the way up to the first priority level. To reduce theamount of traffic on the Pbus for updating interrupt register 450,interrupt image register 452 maintains a copy of what should be ininterrupt register 450. Interrupt image register 452 is updated by theoutput of mask 466 onto internal bus 490, and the pre-update contents ofinterrupt image register 452 are compared with the contents of internalbus 490 by comparator 470. If the contents of internal bus 490 changethe contents of interrupt image register 452, then the contents ofinternal bus 490 are output to the Pbus, otherwise, nothing is output tothe Pbus. In this way, interrupt register 450 and interrupt imageregister 452 are reflections of each other, except for any delay inupdating interrupt register 450. Of course, if microprocessor 12modifies interrupt register 450 internally, it should also updateinterrupt image register 452. The complete interrupt data need not besent each time to microprocessor 12, since microprocessor 12, whennecessary to its operation, can access the registers and masks ofinterrupt filter 198.

The invention has now been described. The above description isillustrative and not restrictive. Many variations of the invention willbecome apparent to those of skill in the art upon review of thisdisclosure. The scope of the invention should, therefore, be determinednot with reference to the above description, but instead should bedetermined with reference to the appended claims along with their fullscope of equivalents.

What is claimed is:
 1. A processor interface circuit, for coupling amicroprocessor operating in tandem to an internal bus which handlesrequests for memory reads, memory writes, and input/output accessesissued by the microprocessor, the processor interface circuitcomprising:a processor bus interface, which maintains a bidirectionaldata path over a processor bus between the microprocessor and theprocessor interface circuit; an internal bus interface, which maintainsa bidirectional data path over the internal bus between the processorinterface circuit and microprocessor request handlers internal to aprocessor system, said microprocessor request handlers including atleast a memory interface circuit which reads and writes a memory inresponse to requests from the microprocessor; a boot address relocator,coupled to said processor bus interface such that said boot addressrelocator accesses an address portion of a memory access request, saidboot address relocator for relocating an address in said address portionwhen a boot indicator is set and said address portion does not point toan address in said memory accessed by said memory interface circuit; arequest pipeline, coupled to said processor bus interface and saidinternal bus interface, for accepting microprocessor requests andacknowledging said requests before said requests are output on saidinternal bus interface when said internal bus interface is not ableimmediately accept said requests; a prefetch queue, coupled to saidprocessor bus interface and said internal bus interface, for fetchingand holding a predicted block of data from said memory, said predictedblock being a block in memory following a block which was requested bythe microprocessor; and an interrupt filter, coupled to said internalbus interface and said processor bus interface, for receiving interruptsignals from said internal bus and passing on to the processor businterrupts from a first class of interrupts, where said first class ofinterrupts have a present effect on program flow of the microprocessor,and storing interrupts from a second class of interrupts, where saidsecond class of interrupts do not have a present effect on said programflow, until said stored interrupts would have an effect on said programflow, if at all.